Urethane elastomers are an important segment of the urethane industry. Urethane elastomers have many potential uses, including cast systems, adhesives, spray coatings, millable gums, and injection molding systems for commercial, automotive and military applications. However unsatisfactory thermal aging characteristics as well as poor hydrolysis and microbial resistance has limited the use of urethane elastomers particularly in applications which require the urethane product to undergo long term environmental exposure before or during its intended use. For example, new air bag technology demands a polyurethane elastomer which combines high-temperature resistance with excellent physical properties, processing parameters, and resistance to hydrolysis, oxygen, and ozone. In particular, for air bag use or any other related use, some of the most important properties are extrudability, high-temperature resistance, low temperature flex, high strength, elongation, low to moderate tensile modulus, good environmental resistance, excellent tear resistance, with an "A" durometer of about 80.
The extrudable urethane elastomers currently commercially available are typically hydroxyl-extended and do not have the thermal resistance necessary to resist the temperature of the gas generator during air bag deployment.
It is believed that there are currently no thermoplastic urethane elastomer compositions available that are urea-extended and that can be extruded with a combination of excellent temperature resistance, physical, and environmental properties. It has been believed in the art that a processable thermoplastic urethane could not be obtained using a diamine chain extender because the resulting urea segments gave the urethane a very high melting point. As such, the polyurethanes could not be processed by typical methods used in processing thermoplastic elastomeric materials, such as extrusion, without decomposition of the urethane.
Extrusion of thermoplastic polyurethane elastomers would be desirable to allow flexibility in the formation of various product shapes and sizes, including air bag bladders and the like. Extrusion also is a less expensive and faster processing method compared to other formation processes such as liquid casting.
Taub, U.S. Pat. No. 3,600,358, discloses a polyurethane elastomer prepared from a 4,4'-methylene bis(cyclohexylisocyanate), neopentyl adipate and aromatic diamine. Following addition of the aromatic diamine to the urethane prepolymer, the mixture is heated and poured into a mold for curing. Taub does not indicate that the urethane can be extruded. Taub also does not disclose or suggest the inclusion of a mixture of diamine materials to improve the extrudability of a urea-extended urethane.
Slagel, U.S. Pat. No. 3,866,242, discloses a protective shield comprising a polyurethane made from a polyester or polyether glycol, a diisocyanatodicyclohexylmethane isocyanate material, and a primary amine such as 4,4'-methylenebis(2-chloroaniline). The polyurethanes are cast between glass plates and cured to form the protective shield. The polyurethane disclosed by Slagel is not elastomeric, as evidenced by the disclosure that the material has a hardness on the "D" scale of 77-80 (col. 3, line 30). Slagel does not indicate that the polyurethane can be extruded. Slagel also does not disclose or suggest the inclusion of a mixture of diamine materials to improve the extrudability of a urea-extended urethane.
Accordingly, it is an object of the present invention to provide a thermoplastic polyurethane elastomer that is extrudable, that has high temperature resistance, and that is strong and durable for use in manufacturing urethane products such as air bags and the like. It is a further object of the present invention to provide a process for making such polyurethanes, as well as a process for extruding such polyurethanes.
Novel features of the invention, together with advantages thereof, will be better understood from the following descriptions in which embodiments of the invention are illustrated by way of examples.